Low profile feedback slot antenna



Feb. 17, 1970 a wA f ETAL 3,496,571

LOW PROFILE FEEDBACK SLOT ANTENNA Filed Jan. 9, 1967 2 Sheets-Sheet 1 FIG'.6A

INVENTORS CARLTON H. WALTER 1-en wc-e' a. Kit- 0W" FIGQ2.

c. H. wALfirER ETA!- LOW PROFILE FEEDBACK SLOT ANTENNA 2 Sheets-Sheet 2 mm Jan. 9, 1967 INVENTORS CARL H. WALTER BYTERE E. K] LCOYNE 3,496,571 LOW PROFILE FEEDBACK SLOT ANTENNA Carlton H. Walter and Terence E. Kilcoyne, Columbus, Ohio, assignors to The Ohio State University Research Foundation Filed Jan. 9, 1967, Ser. No. 608,121 Int. Cl. H01q 13/10 US. Cl. 343-768 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates generally to antennas with frequency scanning beams, and more particularly to antennas with a single frequency scanning beam.

A TEM-line antenna generally comprising a travelingwave antenna integrally formed with a surface wave structure has been described in the patent application of John R. Copeland and Richard A. Kitzerau entitled Integrated Antenna (Surface Wave Antenna), Ser. No. 709,158, filed Feb. 2, 1968 and assigned to the same assignee as the present application. That invention in its most fundamental embodiment comprises a two conductor type of traveling-wave antenna, which contrary to accepted theory is physically positioned with its longitudinal axis adjacent to or embedded in a conducting surface-or ground plane.

The actual two conductor structure utilized in Copeland comprises a coaxial line with periodic gaps in the outer conductor, the structure being terminated in a reactive load. Energy fed into the antenna travels down the line and is reflected by the termination. Thus, there exist two traveling waves on the structure, a forward wave and a backward wave. The spacing of the periodic gaps in the outer conductor is such that for low frequencies the forward wave produces radiation in the backward direction, while conversely, the backward wave produces radiation in the forward direction. As the frequency is scanned from the lower end to the higher end of the antenna bandwidth, the forward beam scans from near endfire, through broadside, to near backfire. At the middle of the frequency bandthe antenna is characterized by a single broadside beam which results from the overlapping of the forward and backward beams. It is also noteworthy that multiple beams, i.e. 2, 4, 6, etc. are possible, the number of beams present being directly proportional to the number of space harmonics present on the structure.

The present invention may be regarded as an improvement upon the Copeland device described, it being a principal object of the invention to provide a TEM-line antenna with but a single, frequency scanning beam.

It may be regarded as another object of the present invention to provide a new and improved antenna structure that may be suitably integrated with the outer surface of a vehicle.

Yet another object of the invention is to provide a TEM-line antenna producing a single but selectable beam direction.

A further object of the present invention is to provide a single frequency scanning beam TEM-line antenna adapted to utilize the skin of a vehicle or aircraft as a radiating-element.

In accordance with the present invention, these and other objects are achieved through utilization of a reentrant technique whereby a feedback path is provided for the traveling wave to re-enter the antenna, thus eliminating one of the two principal beams in the Copelandtype of apparatus.

A fuller understanding of the present invention may now best be gained by a reading of the following detailed specification and by a simultaneous examination of the drawings appended hereto, in which:

FIGURE 1 is an isometric simplified showing of major portions of the antenna structure;

FIGURE 2 is a planar view of the radiating side of the antenna and illustrates in a simplified schematic fashion the electrical connection scheme utilized for the reentry technique;

FIGURES 3, 4, and 5 show typical radiation patterns for a representative antenna constructed in accord with the present invention.

FIGURE 6 is the backside of the antenna of FIGURE 2 illustrating the undercovers; and

FIGURE 6a is a cross-sectional view of one of the undercovers.

Referring now to FIGURE 1 there is illustrated a radiating side view of major portions of an antenna according to the present invention. The coaxial line 10 is constructed to conform to a basic type of two conductor traveling wave antenna. For directional radiation the overall length of the cable will be several wave lengths or longer at the normal operating frequency. Short gaps of the outer conductor 12 are removed to expose the inner conductor 14 to form electrical discontinuities in the cavities 25 cut out of the ground plane 24. The spacing between gaps in the outer conductor approximates a half wave length at the center frequency of the design bandwidth. The end 18 is terminated in a manner which will be discussed in connection with FIGURE 2. However it may be noted that by adjusting the point of termination the exact fundamental frequency may be varied. Electromagnetic energy is fed at end 22.

Referring now to FIGURES 6 and 6a there is illus trated the underside or backside of the antenna of FIG- URE 2. Basically for each of the cavities 25 (of FIGURE 2) formed in the ground plane 24 there is provided an undercover 20. Simply, this undercover 20 is a metallic box covering each of the cavities 25 thereby forming a continuous ground plane for the antenna. The undercovers 20 are electrically and physically attached to the ground plane such as by flanges 15 or soldering. Entering into one side of the undercovers is the coaxial feed line. Specifically, the outer conductor 12 terminates in an opening in one side of the undercover and begins again with an opening on the other side-successively for each of the undercovers. The inner conductor is continuous. In this way the complete backside of the antenna is at ground potential and physically interconnected; Whereas on the radiating side, the inner conductor 14 will be exposed in the cavity 25.

Operationally, the currents in the coaxial line of FIG- URE 1 travel from the source point 22 to the end 18 which will normally be terminated. Any reflected currents would set up standing waves. Gaps along the outer conductor 12 permit energy to be radiated from the coaxial line. The traveling-wave antenna is physically placed in longitudinal contact with a conducting surface, or ground plane, 24. The outer conductor 12 is electrically connected to the ground structure 24 along its entire length and is in extremely good electrical contact at least at the gap points.

Again, operationally, it has been found that the energy radiated by the travelling wave is not shorted out by the surface wave structure. To the contrary, a complete and satisfactory operable antenna is had and the currents are permitted to be propagated. It is believed that the currents normally set up in the outer conductor induce currents in the ground plane (surface wave structure) in the vicinity of the discontinuities. These currents, in turn, are radiated by the ground plane in this specific area.

FIGURE 2 is a planar view of the radiating side of the antenna and illustrates in a simplified schematic fashion the electrical connection scheme utilized for the re-entry technique. In this figure the small cavities 25 present in ground plane 24 are evident, these cavities being adjacent to the open sections of line 10. Exciting energy for the antenna is fed via 26.

In order to eliminate the beam due to a backward (reflected) wave, the travelling wave in cable 10 is fed back into the antenna via terminal 28 and feedback path 29. The feedback path 29 is normally a coaxial connecting cable and is typically of proper length electrically to couple the wave back into the antenna in proper phase relationship. The hybrid junction 27 shown in this embodiment does not per se form a part of the invention, but rather is a standard commercially available item possessing directional coupling characteristics. One such'junction that has been found suitable for use in the present environment is manufactured and sold by Sage Laboratories under the designation Model 752. This particular hybrid junction is of the stripline type. According to the IRE Standards, a hybrid junction is defined as a waveguide (including coaxial transmission line) arrangement with four branches which, when branches are properly terminated, has the property that energy can be transferred from any one branch into two of the remaining three. In common usage, this energy is equally divided between the two branches.

In the present environment the terminal 30 of hybrid junction 27 is matched to a resistive load-strict1y speaking a 3-terminal directional coupler would be preferable. The terminal 31 may be conveniently connected to a generator 32 for transmitting applications or a receiver for receiving applications.

A representative TEM-line antenna was designed in accordance with the foregoing principles to operate in the band of frequencies from 1.2 gHz. to 2.4 gHz. Its operation can be displayed on a K-fi or Brillouin diagram. The coaxial cable used for the antenna was RG 8/u, which has a velocity slowness factor where V =velocity of a point of constant phase alone the line, and c=velocity of light in free space. The low frequency cutoff point on the K- diagram corresponds to KS=0.8 1r where K=free space propagation constant, and S=spacing of the periodic gaps. For 5:10 cm., the cutolf frequency is found as f =1.2 Hz. In like manner, the upper cut off frequency corresponds to KS=1.6 1r and is found to be f =2.4 HZ. In FIGURE 1 dimension A=1O cm. and B=2 cm.

FIGURES 3, 4, and 5 show typical radiation patterns for the exemplary antenna for low, mid, and high frequenciesmore specifically for frequencies of 1225, 1475, and 1795 rnHZ. respectively.

While the present invention has been particularly described in terms of specific embodiments, it will be understood in view of the present disclosure that numerous modifications can be devised Without departing from the present teaching.

Thus, for example, versatility in the re-entrant antenna can be increased by using a simple switch 34 of FIGURE 2 to reverse the direction of power flow. Thus, one could select a beam in either direction with the throw of a switch. Beam direction could also be readily controlled by electronic switching.

It may also be observed that by interchanging the matched termination and the feedback connections at hybrid junction 27 there exists two oppositely directed traveling waves on the antenna. Proper adjustment of the relative phase of the two waves produces the dual beam version of the TEM-line antenna.

Further it is to be noted that both active and passive elements, e.g. a variable phase shifter, an attenuator, or an amplifier, could be incorporated into the feedback path 29. This would permit adjustment of the antenna for opti mum radiation characteristics.

Accordingly the invention is to be broadly construed and limited only by the scope and spirit of the claims now appended hereto.

What is claimed is:

1. A re-entrant, single beam, frequency scanning TEM- line antenna comprising: an inner and outer conductor in longitudinal coaxial relationship, said outer conductor having a plurality of discontinuities spaced a sub-multiple of a wavelength of the fundamental frequency of excitation of said antenna; means for exciting one end of said inner and outer conductors; directional coupling means at the feed end of said inner and outer conductors and coupling said end to a feedback path from the other end of said line, said directional coupling means being adapted to couple only energy flowing toward said junction to reenter said antenna, whereby a traveling wave may be established in a single propagation direction along said line; a surface wave structure having an electrically conductive surface, said inner and outer conductors being positioned in parallel relationship to said conductive surface; and means electrically connecting said outer conductor to said surface, said traveling wave causing currents to be induced in said conductive surface in the area of said discontinuities of said outer conductor.

2. An antenna according to claim 1 wherein said electrically conductive surface of said surface wave structure comprises a structure having at least one planar surface, and wherein said outer conductor of said antenna is in longitudinal contact with said surface.

3. An antenna according to claim 2 in which said directional coupling means comprises a hybrid junction.

4. An antenna according to claim 2 wherein said planar surface is provided with radiating slots adjacent the said discontinuities of said outer conductor.

5. A re-entrant, single beam, TEM-line antenna comprising: a surface wave structure having an electrically conductive surface; a two conductor coaxial line having a plurality of discontinuities in the outer conductor thereof, said outer conductor being in longitudinal contact with said conductive surface; means to establish a traveling wave on said coaxial line; and directional coupling means at feed end of said line coupling at least part of the energy conveyed by said wave in feedback fashion from the opposite end of said line, whereby propagation of traveling waves along said line is in a single direction only.

6. An antenna structure according to claim 5 further including switching means associated with said directional coupling means adapted to selectively reverse the direction of power flow to said antenna and feedback path, whereby beam selection is enabled.

7. A re-entrant, single beam, frequency scanning, line antenna comprising: a two element transmission line having radiating discontinuities therein; means for exciting one end of said transmission line; directional coupling means at the feed end of said line coupling said end to a feedback path from the other end of said line, said 5 directional coupling means being adapted to couple only FOREIGN PATENTS energy flowing toward said junction to re-enter said an- 902,510 12/1953 Germany. tenna, whereby a traveling wave may be established in 1,093,433 11/1960 Germany.

a single propagation direction along said line. 218,886 3/1958 Australia References Cited 5 ELI LIEBERMAN, Primary Examiner UNITED STATES PATENTS F. P. BUTLER, Assistant Examiner 2,303,610 12/1942 Carter 343-734 3,123,827 3/1964 Arnold et al 343-7925 US. Cl. X.R.

3,218,644 11/1965 Berry 343-7925 X 10 343-708, 771, 854 

